Specially devised screens are used in the pipeline industry to reconfigure the fluid flow profile in the pipeline. When used to correct the fluid flow profile in the pipe they are referred to as perforated plate flow conditioners. An example of such a flow conditioner is the invention described in U.S. Pat. No. 5,762,107, which is incorporated herein by this reference. That patent disclosed adding vanes parallel to the flow, both upstream and downstream to the perforated plate. Similarly, U.S. Pat. No. 6,701,963, which is incorporated herein by this reference, discloses a low pressure drop flow conditioner using porous axial vanes.
In operation the perforated plates are installed in the pipeline in front of the flow meter. The perforations (holes) in the plate cause the fluid flow to be reconfigured or readjusted in the radial directions so as to develop a fluid flow velocity profile which is preferred. In some cases this preferred fluid flow velocity profile can be that which is normally seen in a long straight piece of flowing pipe, or can be of a condition which is simply repeatable (can be exactly recreated time after time). The net positive effects of the flow conditioning device is that the flow meter which is located downstream of the flow conditioner operates in a more accurate and repeatable fashion. There are numerous perforated plates used in industry some patented, some public domain. Noise generation is a detrimental aspect of perfroated plates.
When fluid flows past a perforated plate, which can be a disk containing holes of any type of diameter, array, or configuration, noise is generated. The noise generation is a normal physical characteristic of the fluid flow case, but it is a detrimental attribute which can in some cases offset the positive measurement effects of the perforated plate flow conditioner.
The undesirable noise is generated by harmonic interaction between the hole, fluid jets downstream of the screen, a flat spot of the plate on a rear section between the holes, and the location of the impact point of the fluid jets, which is a coalescing point. The physics of noise generation can be understood by reference to one hole pair and the “flat spot” between the two holes. The flow conditioner can be made of any number of holes. At least one hole pair and the accompaning rear flat spot between the holes create the noise phenonenom.
Referring now to FIG. 1(a)–(e), a prior art perforated plate flow conditioner 9 has holes 10. As fluid passes through the holes 10, each hole initiates a high speed stream 11 of fluid at an initiation point, which is the upstream hole inlet edge 12. Prior attempts to solve the noise phenomenon focused efforts at the upstream hole inlet edge 12, but with only marginally successsful results, because the fundamental physcial noise generation phenomenon downstream of the perforated plate was overlooked.
As fluid travels through each hole 10, the fluid accelerates and develops the stream 11 which is bounded by the inside walls of the hole. Upon exit from the hole the fluid streams 11 expand to meet the pipe flow conditions downstream. Exit vortices are generated as the streams 11 exit from the flow conditioner. If the expanding streams 11 are exiting adjacent holes, the point where the adjacent exiting streams touch is a coalescing point 14. The vortices contained within the exiting streams 11 are dynamic in nature, and can therefore generate some acoustic noise of a frequency dependent on the hole diameters and the distance between the holes.
From a downstream side 15 of the perforated plate flow conditioner 9 to the coalescing point 14 is a distance 16 which is a function of, and is dependent on, the fluid flow velocity and the diameter of the stream 10. At the coalescing point 14 some small amount of acoustic energy is generated from the contacting jets. When the distance 16 is at some whole number product of the wavelength distance of the acoustic emmitance of the coalescing point 14, acoustic resonance occurs. The accoustic energy from the coalescing point 14 feeds back to a downstream side 15 flat spot 18 between holes, where it is reflected back to coalescing point 14, but it also disturbs the jet vortices at the hole exit location. The disturbed jets meet at the coalescing point 14, then emit acoustic energy, and the cycle continues. This feedback cycle continues until the acoustic energy becomes detrimental noise, as shown in FIG. 1(f). This noise is detrimental to flow meter performance and is environmentally unacceptable.
Thus, flowmeters such as disclosed in U.S. Pat. No. 6,647,806, which is incorporated herein by this reference, which use a turbulence conditioner for use with transit time ultrasonic flowmeters, suffer from decreased performance due to the noise generated by the flow conditioner.
Numerous patented and unpatented perforated plate flow conditioners, and other types of devices which are used to modify flow in pipe for fluid flow measurment (not all flow conditioning devices are perforated plates) are produced by the following companies:                Akashi        AMCA        Bellinga        Bosch & Herbrard        Gallagher (Savant Measurement)        ISO        K-Lab        Kinghorn        Laws (Elizabeth M. Laws)        K-Lab (Statoil Norway)        Mitsubishi        CPA 50E (NOVA 50E)        Gas DeFrance        PG&E (John Stuart)        Sens & Teule        Spearman        Sprenkle        Stuart C-3        Vortab        Zanker        
Attempts to modify the generation of perforated plate noise by modifying the edge sharpness at the upstream hole inlet edge 12, as shown in FIG. 1(h), have been the only noise elimination attempted solutions to date. Effectiveness of this approach has been only marginal, because the modification of edge sharpness at the hole inlet edge 12 simply changes the distance 16, thereby changing the harmonics acoustic noise generation feedback system—the location of the coalescing point 14 and the wavelength of the emitted noise. When the pipe fluid velocity happens to make the acoustic wavelength equal to the distance 16, noise is again emitted,.although at a new frequency which may not be as detrimental.
Previous attempts to silence perforated plate noise have been only partly successful:                “Whistling Flow Straighteners and their influence on Ultrasonic Flow Meter Accuracy. Authors Dr. H. J. Riezebos, Gasunie, Netherlands, Ing. J. P. Mulder, Gasunie, Netherlands, Ir G. H. Sloet, Gasunie, Netherlands, Ing R, Zwart, Gasunie, Netherlands. In this methodology the researchers focused on only the leading and trailing hole edge contours of the perforated plate. Some reductions in noise pressure level were achieved, but the fundamental noise generation mode is still operationally unabated.        U.S. patent application Ser. 20040055816 by James Gallagher et al., published Mar. 25, 2004, which is incorporated herein by this reference, discloses an apparatus for filtering ultrasonic noise within a fluid flow system. The application states, “the noise filter 410 provides an absorbent element having absorbent material thereon which converts indirect noise propagation into vibration (and, also thereby converting the indirect noise energy into small amounts of thermal energy). The device appears to be similar to a packed muffler, and the absorbent material has apparently had longevity problems.        U.S. Pat. No. 6,533,065 to Zanker, which is incorporated herein by this reference, discloses a noise silencer for use with an ultrasonic meter. The silencer comprises a tubular body having at least two baffles spaced apart from one another. The baffles are preferably formed of an open-cell, reticulated metal foam material that absorbs noise in the ultrasonic range of frequencies under high-pressure operating conditions. However, this silencer, in addition to being expensive, is passive, and converts the noise generated into heat after the fact. That is, it does not deal with the source of the problem. This silencer is prone to self-destruction because the gas velocities in the pipe are large, and damage protruding devices like this device. Finally, this silencer creates a high pressure drop.        Blaine Sawchuk, one of the inventors of the present invention, and Wayne Haner of Winnepeg, Manitoba, Canada, developed various hole edge configurations on perforated plates. Chamfering the downstream edge of the hole 11, as shown in FIG. 1(g), did nothing to noise elimination. Chamfering the upstream hole inlet edge 12, as shown in FIG. 1(h), reduced the flow conditioner noise slightly. Rounding the leading edge of the perforated plate holes increased the noise generation significantly.        
Currently, no device exists to eliminate the source of the noise where it is generated: at the flat spots 18 between the holes on the downstream side 15, thus interfering with the acoustic feedback loop. What is needed is a device that eliminates the flat spots 18.